Method for manufacturing of magnet poles

ABSTRACT

A method for manufacturing sintered magnet poles is described. A vitrifiable base material powder is filled into a mold, the mold is closed with a plate, the mold with the powder is placed in a magnetic field for aligning the powder, the plate is pressed such onto the powder as to establish a compact that holds the alignment in place, and the compact is sintered so as to form a sintered magnet pole. The mold ultimately forms a protective cover of the sintered magnet pole and the plate ultimately forms a base plate of a magnet pole piece. Furthermore, a magnet pole piece is provided which has a magnet pole and a base plate which is fixed to a protective cover so that the base plate and the protective cover surround the magnet pole. The base plate and/or the protective cover of the magnet pole piece has at least one element that provides a geometrical locking of the magnet pole to the base plate and/or the protective cover.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of European Patent Office applicationNo. 07014331.8 EP filed Jul. 20, 2007, which is incorporated byreference herein in its entirety.

FIELD OF INVENTION

The present invention relates to a method for manufacturing magnetpoles.

BACKGROUND OF INVENTION

Permanent magnets are used to an increasing extent in large electricalmachines, for instance in motors, generators and other such machines.This is due to the increased efficiency and robustness compared withelectrical excitation. In particular, rare-earth magnets, primarilybased on neodymium iron boron (abbreviated as NdFeB), have turned out toprovide a very high energy product and are therefore very useful incompact machinery.

However, some problems in relation to the practical application remainunsolved. The best magnet materials corrode very easily, need a highdegree of protection and are also rather brittle and cannot safely befixed by bolting alone. Furthermore, the known manufacturing methodscomprise a long series of steps, some of which are expensive and involvewastage of costly materials.

The traditional method of mounting permanent magnets on, for example, alarge generator rotor comprises extensive surface protection on theindividual magnets and gluing the magnets to the rotor rim. Furthermore,it is necessary to wrap the completed rotor with glued-on magnets with afibreglass bandage.

This method contains several difficulties. The surface protection isextensive and due to new technologies it is not proven over a lifetimeof, for example, 20 years. The magnets cannot be magnetised in situ.This means that all work is done with magnetised parts which requirespecial tools and stringent control of the work to avoid hazardoussituations. Once mounted on the motor and covered by a fibreglassbandage the magnets cannot be removed for re-magnetising in the case ofan irreversible demagnetisation event.

SUMMARY OF INVENTION

In order to overcome these difficulties solutions have been developedwhereby magnets are manufactured as complete pole pieces. In a polepiece one or more magnets are glued to a steel base plate and arecovered with a protective cover. The protective cover is typically madeof stainless steel. The manufacturing of such a pole piece requires theprior manufacturing of finished permanent magnets for subsequentcompletion as pole pieces.

One of the key processes for the manufacturing of neodymium iron boronbased magnets is sintering. Sintered neodymium iron boron based magnetsachieve their coercivity by virtue of a neodymium rich phase at grainboundaries which acts to produce liquid phase sintering, smooth theboundaries and hence prevent nucleation of reverse magnet domains.

The state of the art processing route for sintered neodymium iron boronbased magnets starts with a cast ingot of rare earth neodymium materialmixed with iron and boron in an Nd₂Fe₁₄B composition. The as-cast ingotis first broken into a powder. This is achieved most conveniently byexposing the ingot to hydrogen which is absorbed at the surface andenters the material in the spaces between the atoms and causes thematerial to expand. The differential expansion generates stress in theingot and the alloy breaks down into a fine powder. This process isknown as Hydrogen Decrepitation (HD). The HD powder is then broken upfurther by a jet milling stage which reduces the particle size to around5 mm. When the alloy is in powder form it is very flammable and must behandled under an inert gas.

When the powder has been broken down to such a fine size each particleof powder is a single crystal which can be aligned in a magnetic field.This alignment is held in place by pressing the powder into a greencompact which is about 60% dense. The compact is then heated in a vacuumto about 1.060° C. for one hour. During the heating stage the hydrogenexits the material and is pumped away. When held at about 1,060° C. forone hour sintering occurs and the compact densifies with the assistanceof the liquid formed by the melting of the neodymium rich phase. Aftersintering the magnets are quenched and then heat treated in order toachieve optimum magnetic properties.

The magnet must then be machined to the final dimensions required forthe intended application. Due to the large degree of shrinkage thatoccurs during sintering, which is greater in the direction of alignment,it is not possible to press compacts that will shrink to the exactrequired size. The machining is a very expensive operation and,particularly for small magnets, a large proportion of the material mayneed to be machines away.

Due to the highly reactive nature of the neodymium rich phase, themagnets tend to corrode very rapidly, particularly in moistenvironments. Therefore, the next stage in the processing is to providea protective barrier on the surface of the magnets. This is usually donewith a nickel coating. It is also possible to use aluminium, zinc andepoxy resin as a coating.

In a next step the magnets are mounted in a pole piece. One or moremagnets are glued to a steel base plate and are covered with aprotective cover. The protective cover can be made of stainless steel,for example. In order to ensure that the magnets will not move insidethe protective cover, in case the glue joint with the base plate givesway, the inside of the protective cover is filled with a fillingmaterial, for example, epoxy resin or silicon rubber. Provided that thecover does not allow the diffusion of water vapour and the fillingmaterial surrounds the magnets completely, a high-degree corrosionprotection of the magnets is not required.

This method more or less eliminates the difficulties of traditionalmagnet mounting. Expensive surface protection is not required, themagnets can be magnetised after mounting in the pole pieces and the polepieces can be removed for re-magnetising in case of an irreversibledemagnetisation event. Some practical difficulties remain, however.Firstly, the process has many steps, some of which are expensive andinvolve the removal of expensive magnet material by grinding. Secondly,the long process involves numerous steps that are critical for thequality of the finished product, for example gluing and other steps.Thirdly, the fixing of the protective cover can be difficult withoutdamaging the filling material.

It is therefore an objective of the present invention to provide anadvantageous method for manufacturing a permanent magnet pole. Further,it is an objective to provide an advantageous magnet pole piece.

This objective is solved by a method for manufacturing magnet poles asclaimed in an independent claim and a magnet pole piece as claimed in anfurther indepenent claim. The depending claims define furtherdevelopments of the invention.

The inventive method for manufacturing sintered magnet poles comprisesthe following steps: a vitrifiable base material powder is filled into amould that ultimately forms a protective cover of the finished magnetpole. The mould is closed with a cap, which is implemented as a plateand which ultimately forms a base plate of a magnet pole piece. Themould with the powder is placed in a magnetic field for aligning thepowder. The cap or plate is pressed such onto the powder as to establisha compact that holds the alignment in place. In a last step the compactis sintered so as to form a sintered magnet pole.

The inventive method allows the sintering of the magnet in situ directlyin a component that can subsequently form the mounting base andprotective cover of the finished magnet pole. The inventivemanufacturing method has several advantages compared to other knownmethods. Firstly, several steps of the known methods for themanufacturing of magnet poles are eliminated. Especially the magnetrequires no expensive machining and there is no separate gluing process.Furthermore, no expensive magnet material is removed by grinding.Moreover, no corrosion protection of the magnet is required since it canbasically be totally enclosed before being exposed to any humidity.

The sintering may be performed by heating in vacuum. By heating themould with the compact in vacuum any included hydrogen is escaping fromthe compact during the sintering of the compact. The sintered magnetpole can be quenched. Moreover, the sintered magnet pole can be heattreated in order to achieve optimum magnetic properties.

A suitable vitrifiable base material for sintering may be neodymium ironboron based powder. Such a powder can be provided by conventional stepsincluding casting, decrepitation and milling, resulting in the neodymiumiron boron based powder suitable for sintering.

The mould can be fixed to the plate before or after quenching or heattreatment, especially to ensure corrosion protection of the magneticmaterial. For example, the mould may be fixed to the plate by welding orsoldering. It is advantageous if the mould ultimately forms a protectivecover of the sintered magnet pole. Furthermore, the plate ultimatelyforms a base plate of a magnet pole piece. Alternatively, it may alsoform the base plate of the finished magnet pole.

Moreover, a filling material can be introduced between the magnet poleand the mould. The filling material may be introduced after fixing themould to the plate by vacuum injection. As filling material for exampleepoxy resin or silicon rubber can be used. The filling material providesan additional protection of the magnet pole.

Generally, the mould can be manufactured of magnetic or non-magneticmaterial. It can especially be manufactured of stainless steel or anyother suitable preferably non-magnetic material. However, the mould mayalso be made of a magnetic material, provided the wall thickness is onlya small fraction of the magnet pole width.

The mould may be in its final shape before the sintering of the compact.Alternatively, it may be only in a rough shape. It is then pressed intoa die during the sintering of the compact to acquire its final shape.Furthermore, the cap or plate can be machined to its final thicknessbefore the sintering or it can be machined after sintering or after heattreatment, especially to ensure proper flatness and other dimensionaltolerances.

Preferably the protective cover and/or the base plate are fitted withsuitable holes, grooves and/or protrusions that engage the magnetpowder, and subsequently the finished sintered material, in order toestablish a geometrical locking of the finished magnet to the protectivecover and/or the base plate.

The inventive method may be used for manufacturing magnet poles whichmay be used for an electrical machine. The electrical machine may be,for example, a generator or a motor. The inventive method is preferablyused for manufacturing magnet poles for wind turbine generators,especially large wind turbine generators.

The inventive magnet pole piece comprises a magnet pole and a base platewhich is fixed to a protective cover so that the base plate and theprotective cover surround the magnet pole. It is characterised in thatthe base plate and/or the protective cover comprises at least oneelement that provides a geometrical locking of the magnet pole to thebase plate and/or the protective cover. The element may be a hole, agroove or a protrusion. The magnet pole can additionally be fixed to thebase plate, for instance by gluing.

It is advantageous if the protective cover is equipped with at least oneprotrusion on each side which fixes the magnet pole to a particularposition inside the protective cover. In this case it is especiallypossible to renounce gluing the magnet pole to the base plate. Thissimplifies the demounting of the magnet pole, for example forre-magnetising in case of an irreversible demagnetisation event.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, properties and advantages of the present inventionwill become clear from the following description of embodiments inconjunction with the accompanying drawings.

FIG. 1 schematically shows a mould filled with neodymium iron boronbased powder in a sectional view.

FIG. 2 schematically shows the mould of FIG. 1 after magnetic alignmentof the powder and covering it with a base plate in a sectional view.

FIG. 3 schematically shows a magnetic pole piece after sintering in asectional view.

FIG. 4 schematically shows the magnetic pole piece of FIG. 3 which isadditionally equipped with a filling material in a sectional view.

FIG. 5 schematically shows an alternative magnetic pole piece in asectional view.

DETAILED DESCRIPTION OF INVENTION

A first embodiment of the present invention will now be described withreference to FIGS. 1 to 4. At first the inventive method formanufacturing sintered magnet poles requires a suitable base materialfor sintering. This base material, for example, may be neodymium ironboron based powder 2 suitable for sintering, i.e. vitrifiable neodymiumiron boron based powder. It can, for example, be provided byconventional steps including casting, decrepitation and milling as ithas been described in the introductory part of this specification. Thepowder 2 is filled into a mould 1 that will, in the present embodiment,ultimately form the protective cover. This is schematically shown inFIG. 1, which shows the mould 1 which is completely filled with theneodymium iron boron based powder 2. The base powder 2 comprises singlecrystal powder particles with randomly orientated magnetic dipoles 3.The random orientation of the magnetic dipoles of the powder isindicated by arrows 3. The mould 1 comprises an opening 8 which isclosed with a valve 6.

In a next step the mould 1 is closed with a cap in the form of a plate 5that will, in the present embodiment, ultimately form the base plate ofthe finished magnet pole. Then the mould 1 with its powder fill 2 isplaced in a magnetic field for the alignment magnetic dipoles of thepowder particles. Preferably the plate 5 is firmly pressed onto thepowder fill 2 to establish a compact 14 that holds the alignment inplace. The result is schematically shown in FIG. 2 where the mould 1which is closed with the plate 5 and filled with the compact 14 made ofthe pressed and aligned neodymium iron boron based powder 2 is shown ina sectional view. The aligned magnetic dipoles of the compact 14 areindicated by arrows 4. One can see in FIG. 2 the mould 1, which is alsoshown in FIG. 1, equipped with an opening 8. The opening 8 is stillclosed with a valve 6.

The mould 1 and/or the plate 5 can be made of stainless steel, forexample. The mould 1 with the compact 14 is heated in a vacuum toeliminate any hydrogen remaining from an HD process and sintering of thecompact 14 to form the sintered magnet pole 7. After sintering, theresulting magnet pole is optionally quenched and then heat treated inorder to achieve optimum magnetic properties.

During the sintering process the former neodymium iron boron basedpowder compact shrinks and forms the magnet pole 7. One can see in FIG.3, which schematically shows the magnet pole 7 obtained after sinteringthe compact 14, that the shrinkage is greater in the direction of thealignment of the magnetic dipoles 4. Due to the shrinkage a hollow space10 occurs between the magnet pole 7 and the protective cover 1. Comparedto FIG. 2, the magnet pole 7 in FIG. 3 is turned so that the plate 5 isnow on the bottom side. The magnet pole 7 in FIG. 3 is fixed to theplate 5 caused by the sintering process. Moreover, the magnet pole 7comprises aligned magnetic dipoles 4.

After quenching and heat treatment the mould 1 is welded to the plate 5.The plate 5 then forms the base plate of the magnet pole 7 to which themould 1 as a protective cover is fixed. FIG. 3 shows a weld seam 9 bywhich the mould 1 is welded to the plate 5. The weld seam 9 is locatedat the connection between the mould 1 and the plate 5. Alternatively,the protective cover may be fixed to the base plate 5 before quenchingand heat treatment. Instead of welding, soldering or any other suitablemethod for fixing the protection cover to the base plate may also beapplied.

The opening 8 in the protective cover 1, which is shown in FIG. 3, makesit possible to evacuate the hollow space 10 and to fill it with asuitable filling material 11. FIG. 4 schematically shows a magnet polepiece in which the hollow space 10 is filled with a filling material 11in a sectional view. Epoxy resin or silicon rubber may be used as afilling material 11, for example. After filling the hollow space 10 withthe filling material 11 it is possible to close the opening 8 again withthe valve 7. Alternatively, the opening 8 may be closed by means of afilling material 11, as shown in FIG. 4.

A second embodiment of the invention will now be described with respectto FIG. 5. Elements of the present embodiment which correspond toelements of the first embodiment are designated with the same referencenumerals and will not be described again to avoid repetition.

Regarding the individual steps of the applied inventive method it isreferred to the first embodiment. FIG. 5 schematically shows analternative finished magnetic pole piece in a sectional view. One cansee in FIG. 5 a base plate 5 to which a magnet pole 7 is fixed. Themagnet pole 7 comprises aligned magnetic dipoles 4. Further, aprotective cover 1, which may have been used as mould during themanufacturing process as described in the first embodiment, is welded tothe base plate 5 so that the magnet pole 7 is surrounded by the baseplate 5 and the protective cover 1. Instead of welding also soldering orany other suitable method for fixing the protective cover to the baseplate can be applied.

In this embodiment a mould that ultimately forms the protective cover 1is used which comprises protrusions 13 at its inner surface. The baseplate 5 is also equipped with protrusions 12 on the surface which is incontact with the neodymium iron boron based powder inside the protectivecover 1. The protrusions 12, 13 provide a geometrical locking of thefinished magnet pole 7 to the protective cover 1 and to the base plate5.

Instead of protrusions 12, 13 the protective cover 1 and/or the baseplate 5 may be equipped with holes, grooves or other suitable means thatengage the magnet powder 2 and subsequently the finished sintered magnetpole 7 in order to establish a geometrical locking of the finishedmagnet pole 7 to the protective cover 1 and/or the base plate 5.

The magnet pole 7 is fixed to the base plate 5 inside the protectivecover 1 by means of protrusions 12 of the base plate 5 and caused by thesintering process. The space between the protective cover 1 and themagnet pole 7, which is caused by shrinking of the magnet pole 7 duringthe sintering process, is filled with a filling material 11. The fillingmaterial 11 may be, for instance, silicon rubber or epoxy resin. Theprotective cover 1 comprises a hole 8 through which the filling material11 is injected into the space between the protective cover 1 and themagnet pole 7. After injecting the filling material 11 the opening 8 isclosed by means of a valve 6.

It is alternatively also possible that the protective cover 1 in bothembodiments may only be in a rough shape and acquires its final shape bygrinding at the end or by pressing it into a suitable die during thesintering process. Moreover, the base plate 5 may be machined to itsfinal thickness before sintering or it may be finished after sinteringto ensure proper flatness and other dimensioned tolerances followingheat treatment.

In summary, the invention discloses a method for manufacturing permanentmagnet pole pieces which simplifies the manufacturing process andeliminates several costly steps. The invention further provides anadvantageous magnet pole piece.

The invention claimed is:
 1. A method for manufacturing a sinteredmagnet pole, comprising: providing a mould having a primary opening anda side opening which is closed with a valve; filling a vitrifiablerandomly oriented base material powder into the mould through theprimary opening; closing the primary opening with a plate; placing themould with the randomly oriented powder into a magnetic field to alignthe randomly oriented powder into aligned powder; pressing the plateonto the aligned powder to establish a compact that holds the alignmentof the aligned powder in place; sintering the compact while the compactis oriented over the plate inside the mould to form a sintered magnetpole that adheres to the plate as a result of the sintering process,wherein a gap is formed between the sintered magnet pole and the moulddue to shrinkage of the compact during sintering; opening the valve;introducing a filling material into the gap through the side opening;and using the protective cover and base plate as part of the finalproduct, wherein the mould ultimately forms a protective cover of thesintered magnet pole and the plate ultimately forms a base plate of amagnet pole piece.
 2. The method as claimed in claim 1, wherein thesintering is performed by heating in vacuum.
 3. The method as claimed inclaim 2, comprising quenching the magnet pole.
 4. The method as claimedin claim 2, comprising heat treating the magnet pole.
 5. The method asclaimed in claim 2, wherein the randomly oriented vitrifiable basematerial powder is a neodymium iron boron based powder.
 6. The method asclaimed in claim 1, comprising quenching the magnet pole.
 7. The methodas claimed in claim 1, comprising heat treating the magnet pole.
 8. Themethod as claimed in claim 1, wherein the randomly oriented vitrifiablebase material powder is a neodymium iron boron based powder.
 9. Themethod as claimed in claim 1, comprising fixing the magnet pole to theplate before or after quenching or heat treatment.
 10. The method asclaimed in claim 9, wherein the mould is fixed to the plate by weldingor soldering.
 11. The method as claimed in claim 9, wherein the fillingmaterial is introduced by vacuum injection after fixing the mould to theplate.
 12. The method as claimed in claim 1, wherein the mould is in itsfinal shape before the sintering of the compact.
 13. The method asclaimed in claim 1, comprising pressing the mould that is in a roughshape inside a die during the sintering of the compact to acquire itsfinal shape.
 14. The method as claimed in claim 1, comprising machiningthe plate to its final thickness before or after sintering or after heattreating the magnet pole.
 15. The method as claimed in claim 1,comprising fixing the magnet pole to the plate.
 16. The method asclaimed in claim 15, wherein the magnet pole is fixed to the plate withglue.
 17. The method as claimed in claim 1, comprising securing themagnet pole in position with at least one element of the base plate orthe protective cover that provides a geometrical locking of the magnetpole to the base plate or the protective cover.
 18. The magnet polepiece as claimed in claim 17, wherein the element is a hole, a groove ora protrusion and wherein the magnet pole is fixed to the base plate.